1. Field of the Invention
The present invention relates to an optical waveguide and a method of fabrication thereof.
2. Description of the Related Art
In recent years, under circumstances where the trend of Internet toward broadband moves forward, it is necessary to reduce a large amount of the cost of devices for optical communication for wide spread use of access such as FTTH. Optical transmitter and receiver modules to convert light to electrical signals are used in terminals of equipment for optical communication as a device for optical communication. In order to bring this optical transmitter and receiver module down in size and cost, there is proposed a method in which an optical waveguide, being a part within the module, is formed from an organic polymeric material (Nobuo Miyadera, “Polymeric materials for an optical waveguide”, Optical Alliance, 1999, no. 2, p. 13).
For example, a lower clad layer is formed on a substrate, and on this clad layer, an optical waveguide layer consisting of an organic polymeric material is formed. In this optical waveguide layer, a pattern is formed and an unnecessary portion is eliminated by reactive ion etching (RIE) and ultraviolet (UV) irradiation using photolithography. On the optical waveguide layer thus formed, an upper clad layer is formed. In many case, the lower clad layer and the upper clad layer are also formed from an organic polymeric material.
When a core layer to be an optical waveguide layer, and a lower clad layer and an upper clad layer are formed from photo-curable (ultra violet curable) resin, it is necessary to control the amount of ultraviolet light irradiated with high accuracy since the refractive indexes of the respective layers vary depending on the amount of ultraviolet light irradiated. However, the present inventors have found a problem that when curing the clad layer or the core layer by irradiating ultraviolet light, the clad layer or the core layer was affectedly a thickness of a substrate and the like and the refractive index of the cure clad layer or core layer varied after curing.
For example, when irradiating the ultraviolet light to the upper clad layer to cure it, part of the ultraviolet light irradiated to the upper clad layer passes through the lower clad layer, penetrates into the substrate, is reflected by the underside of the substrate, passes through the lower clad layer again and reaches the upper clad layer. Since the ultraviolet light reflected within the substrate thus reaches the upper clad layer, curing of the upper clad layer is also advanced by this reflected ultraviolet light. Therefore, an amount of the ultraviolet light reflected at the substrate and reaching the upper clad layer varies due to the substrates having different thicknesses and thereby an amount of the ultraviolet light irradiated to the upper clad layer. As a result of this, a state of the cured upper clad layer varies and unevenness of its refractive index results. Thus, there arises a problem that the clad layer or the core layer is affected by variations in thickness of the substrate and the like and unevenness of the refractive index of the clad layer or the core layer results and therefore it is impossible to set the refractive index of each layer at a predetermined level as designed.